General sewage, food waste and livestock excretion contain much phosphorus and nitrogen that contaminate a river or seawater through eutrophication. However, since there is not present an apparatus for treating sludge equipped with separate step for treating such phosphorus and nitrogen efficiently, there is a problem that sludge containing much phosphorus and nitrogen is discharged thereby contaminating a river or seawater seriously.
In conventional advanced process for treating foul water or wastewater, some sludge sedimented at the bottom of a first sedimentation tank are transferred to a dewatering device, a solution located at the top is transferred to a biological reactor that enhances proliferation of microorganisms by dissolving oxygen in air, and BOD (Biochemical oxygen demand) is removed and phosphorus and nitrogen that greatly influences eutrophication are removed by action of microorganisms in the biological reactor. There is a problem that since sludge is digested under a condition of interrupting oxygen supply in an anaerobic digester included in usual process for treating foul water or wastewater, phosphorus present in a solution through being discharged again by microorganisms that took it previously, and nitrogen added from sludge during digestion and dewatering steps are transferred to the biological reactor together with dewatered solution, and thus excess nitrogen and phosphorus are present in main step.